The diameter of a single crystal is controlled to a set point diameter during pulling of the single crystal from a melt contained in a crucible and which forms a meniscus at a phase boundary on the edge of the single crystal, the meniscus having a height which corresponds to the distance between the phase boundary and a level of the surface of the melt outside the meniscus, comprising repeatedly: determining the diameter of a bright ring on the meniscus; calculating a diameter of the single crystal while taking into account the diameter of the bright ring and the dependency of the diameter of the bright ring on the height of the meniscus and on the diameter of the single crystal itself; and calculating at least one manipulated variable for controlling the diameter of the single crystal on the basis of the difference between the calculated diameter of the single crystal and the set point diameter of the single crystal.